1. Toast the pecans for 10 minutes in a 325 oven. Cool, and pulse in a food processor with the Mexican chocolate until it forms a nut butter. Scrape into a saucepan and add the bittersweet chocolate, cinnamon, and half-and-half. Cook over medium heat until steaming, whisking thoroughly, then turn off the heat, cover and let stand 20 minutes.
  2. In a double-boiler top, whisk together the egg yolks and sugar until thoroughly combined, then whisk in the chocolate-nut mixture. Set over the water and whisk frequently until it thickens, about 5 minutes (180 degrees).
  3. Cool the custard over ice, whisking regularly until completely cool. Stir in the rum and freeze according to the machine's directions.

source: Fiesta at Rick's - Rick Bayless